Ethylene/alkyl ester copolymers are well-known in the art and a large number of such copolymers are available commercially, for example ethylene/methyl acrylate and ethylene/ethyl acrylate copolymers. In the past one of the problems associated with continuous processes for preparation of such compositions in stirred tank reactors was the tendency for large amounts of polymeric deposits to form on the inner walls of the reactors. This buildup, which consists of highly crosslinked insoluble polymer, i.e. gel, adversely affected operation of the reactor. In addition, particles of the gel were invariably introduced into the product which tended to compromise the quality of the copolymer produced. It has recently been found that this problem can be alleviated by performing the copolymerization in the presence of small amounts of methanol or acetone, as disclosed in U.S. Pat. No. 5,028,674 and U.S. Pat. No. 5,057,593. Due to the chain transfer activity of methanol and acetone, however, in some cases the polymeric products thus obtained are of extremely low viscosity, so low in fact that they are unsuitable for many applications.
Methods are known for increasing the viscosity of ethylene/acrylic acid ester copolymers, for example by partial crosslinking, as disclosed in U.S. Pat. No. 4,987,199, but these prior art methods result in the formation of substantial amounts of gel in the product. An in-line process for reducing melt index is also known and is disclosed in U.S. Pat. No. 3,988,509. However, use of that process can result in nonhomogeneous distribution of highly crosslinked gel in the polymer matrix. In addition, there are safety concerns associated with the use of in-line addition of highly reactive species such as peroxides in process streams wherein the possibility exists that polymer movement through the treatment zone can be stopped due to a process upset resulting in localized heating and polymer decomposition.
There is thus a need in the art for a method whereby the viscosity of ethylene/alkyl ester copolymers can be increased without concomitantly raising the gel content of the polymer or producing products which contain domains of highly crosslinked gel within the polymer matrix.